In response to increasingly complex antenna illumination requirements, particularly as to radar systems, much of the developmental work respecting antenna systems over the last two decades has been focused on phased array antenna systems. A phased array is essentially the special case of an antenna which, instead of having a continuous aperture, has an aperture composed of a number of individual radiating elements. The number of such radiating elements in the overall phased aperture may range from a few to many thousands, but typically will be at the upper end of that range. Although construction of an aperture out of many radiating elements involves a substantial increase in electronic and mechanical complexity, as compared to a single continuous aperture, such an array antenna system offers the very important advantage that the amplitude and phase of each radiating element may be controlled in such a way as to produce a desired aperture illumination distribution with substantial flexibility and accuracy. By varying the aperture illumination function a considerable variety of beam patterns may be realized from such a phased array antenna system and certain non-desirable effects, such as large sidelobe levels, may be minimized. For examples of such phased array antenna systems and descriptions of how they work, reference is made to the following sources: Radar Handbook, edited by Merrill Skolnik, 2nd edition, published by McGraw Hill, Inc. (1990) (see Chapter 7 entitled "Phased Array Radar Antennas" by Theodore Cheston and Joe Frank.); Antenna Theory: Analysis and Design, by Constantine A. Balanis, published by Harper & Row, Inc. (1982) (see Chapter 6 entitled "Arrays: Linear, Planer, and Circular"); Antenna Theory and Design, by Warren Stutzman and Gary Thiele, published by John Wiley & Sons, Inc. (1981) (see Chapter 3 entitled "Arrays", especially section 3.7 entitled "Phased Arrays").
Additionally, the flexibility to control the aperture illumination distribution confers another important advantage to phased-array systems, in that the illumination function may be controlled electronically, and thus rapidly, by changing the phase and amplitude of signals to or from the individual array elements. This characteristic is significant in allowing such an antenna to operate in a multi-function way, such as the performance of interlaced surveillance while simultaneously tracking a variety of targets. Changing both phase and amplitude in prescribed ways produces beam-steering and changes in the antenna pattern.
Array antennas are classified as either active or passive. An active array will contain phase shifting and amplification devices behind every element (or group of elements) of the antenna. Passive arrays, on the other hand, are driven from a single feed point. Active array antennas generally offer more flexibility and are capable of higher power than either passive array or conventional antennas.
With an active array antenna, electronic beam-steering of the antenna beam is normally accomplished through the use of an electronically controllable phase shifter at each array element. Additional versatility in such beam-steering may be accomplished through the use of both amplitude and the phase control elements at each array element.
A typical embodiment of such phase and amplitude control elements for placement at an array element of an active array antenna is illustrated in FIG. 1. In that figure, the phase shifting and amplification elements, along with transmit/receive switching elements, which will hereafter be collectively designated as the transmit/receive module, are shown enclosed by dashed-line box 10. This transmit/receive module 10, along with means for controlling the several elements of the transmit/receive module, designated as module control means 15, represent the electronic elements and interconnecting circuitry which will be located at each antenna element of an active array antenna system, and are collectively designated as antenna array elemental subsystem 20.
As will be seen in FIG. 1, in the transmit mode an RF signal enters the antenna array elemental subsystem 20 at RF feed 25 and is thereafter operated on by phase shifter 30, the output thereof passing through transmit/receive switch 35, which will be set at the transmit position, to transmitter gain control 40. That transmitter gain control 40 operates in conjunction with phase shifter 30 to establish the parameters of the beam emitted by the antenna array element driven by this transmit/receive module. The output of transmitter gain control 40 is fed to power amplifier 45 and thus, via circulator 50, to an antenna element 55 from which the transmitted beam is radiated.
In the receive mode, a signal received at antenna element 55 is routed via circulator 50 to transmit/receive switch 60, which will be set in the receive position, and thence to the input of low noise amplifier 65. From the output of low noise amplifier 65, the received signal traverses receive gain control 70, transmit/receive switch 35, which will be set in the receive position, phase shifter 30 and thence exits the antenna array element subsystem 20 via RF feed 25. Receive gain control 70 and phase shifter 30 cooperate in the receive mode to control the receive beam pattern so as, for example, to establish the placement of pattern nulls in the direction of interference sources.
As indicated by directional arrows on signal lines from module control means 15, the electrical settings for each of the electrically adjustable elements of transmit/receive module 10 for a given transmitted or received signal are controlled by module control means 15.
It will be understood that, in the receive mode, low noise amplifier 65, receive gain control 70 and phase shifter 30 may be operated either at RF or IF, with a down converter being introduced between antenna element 55 and the input to low noise amplifier 65 in the case of operation at IF.
As described above, each transmit/receive "T/R") module, when viewed at a functional level, is comprised of RF circuitry and control circuitry. The complement and arrangement of the control circuitry for a T/R module depends on the beam steering architecture used. Three beam steering architectures are illustrated in FIGS. 2A, 2B and 2C. Using FIG. 2A as exemplary, the various control elements shown therein are functionally defined as follows:
RF Device Control 115 generates control signals for the RF circuitry within the T/R modules--i.e., Phase Shifter 30, Gain Controls 40 and 70, Power Amplifier 45, Low Noise Amplifier 65 and T/R Switches 35 and 60; PA1 Array Interface 120 represents the interface between system elements located at the array radiating elements and other system elements; PA1 RF Device Phase and Amplitude Correction 125 establishes compensation for internal errors of T/R module RF Circuitry from changes in operating temperature of a module and errors related to frequency changes, for all phase and gain states, and interactions therebetween; PA1 Beam Shaping and Array Correction 130 accepts computed phase and gain settings from a central processor and modifies such settings for individual T/R modules by array correction term and tapering/beam spoiling term; PA1 Beam Steering Computation 135 computes desired phase and gain settings based on such input parameters as: Beam Pointing, Module Row/Column Location, Antenna Type (Planar/Cylindrical) and Feed Type (Space/Corporate); and PA1 System Interface 140 establishes the interface between the array components downstream from that interface and the system providing a signal to the array antenna, such as a radar system or a communications system. PA1 The correction terms are technology dependent, process dependent and temperature dependent. This being the case, any change in either technology, process or temperature would necessitate re-calibration of the antenna in the field, resulting in high-maintainability costs and relatively lower system availability. PA1 Since correction data are centralized, the individual T/R modules will be interchangeable if, and only if, correction data is also interchangeable. To achieve such a goal would substantially increase house-keeping costs and also reduce system availability. In practice, modules will not be freely interchangeable. PA1 To achieve the cost effectiveness of centralized processing, it is essential that magnetic disk storage be used for centralized storage of correction data. With such an arrangement, a considerable amount of time would be lost in data manipulation and beam position updates, thus defeating the very purpose of phased-array antennae. As will be well known, phased-array antennae are employed in high performance systems such as radar primarily to exploit the faster look-back, scan-back and look-forward (scan forward) capabilities of such systems which cannot be realized in mechanically scanned antennae. PA1 The functional relationship, F(), between various parameters is of the following form: PA1 As will be seen, the phase setting and gain setting relationships are inter-dependent and have complex relationships. In order to devise any meaningful curve fitting type algorithm, one needs a large database each time a change in technology or process occurs. Since T/R module technology continues to evolve, such a large database invariant to both process and technology changes is not available and is unlikely to be available for a long time to come.
It will be understood that, in an active-array antenna system, RF device phase and amplitude corrections should preferably be co-located with the RF circuitry to which such correction factors are to be applied. With such co-location, the T/R module becomes a stand-alone device, independent of antenna type or application and invariant to technology or process changes. As will be seen, each of the illustrated beam-steering architectures incorporates this desired co-location. Beyond that common trait, however, the three illustrated beam-steering architectures differ in many important respects.
The beam-steering architecture of FIG. 2A incorporates centralized beam position computation, correction and correction-data storage. While this architecture has an advantage in that the module dependent corrections can be combined with the antenna dependent corrections whenever field calibration of the antenna is done, its many disadvantages strongly outweigh that advantage. Among the more important disadvantages of this architecture are:
To summarize with respect to the centralized beam steering architecture of FIG. 2A, the advantage of combining antenna frequency dependent correction terms with module dependent correction terms during field calibration of the antenna must be weighed against the substantial disadvantages of poor system response time, high maintainability costs and poor system availability.
In the beam-steering architecture of FIG. 2B, RF corrections (module corrections) are achieved using a curve-fitting type algorithm. Each element "knows" its position in the array, and computes its phase and gain settings using that position, its unit pointing vector and correction factors in the computation. While this architecture overcomes many of the disadvantages of the architecture of FIG. 2A, it introduces new disadvantages, as described below:
.PHI..sub.s =F(.PHI..sub.c, G.sub.s, f.sub.c, T) PA2 G.sub.s =F(G.sub.c, .PHI..sub.s, f.sub.c, T) PA2 .PHI..sub.s =Phase setting as seen by the module, PA2 G.sub.s =Gain setting as seen by the module, PA2 .PHI..sub.c =Control phase, a system parameter, PA2 G.sub.c =Control gain, a system parameter, PA2 f.sub.c =Control frequency, a system parameter, PA2 T=Module temperature which is a function of environment, module location in the array, and the transmit or receive mode of operation.
where,
As is well known, because of the usually very large number of array elements in a phased-array antenna system, the cost of the T/R modules, which must be multiplied by the number of array elements, is critical in determining economic feasibility for a phased-array antenna application. The T/R module cost drivers are: RF circuitry cost, digital control circuitry cost and the RF shielded connector cost. In the beam-steering architecture of FIG. 2B, each array element will contain RF circuitry, digital arithmetic computation and correction circuitry and MxN addressing, where M is the number of rows and N is the number of columns in the array. For the completely distributed architecture of FIG. 2B, each element must have log.sub.2 M + log.sub.2 N connector pins-" . " designating a least integer bigger than (.). As will be apparent, such a distributed architecture arrangement will result in a higher module cost because of the distributed arithmetic computation circuitry and the number of interfacing connector pins.
Accordingly, it is an object of the invention to realize the operational advantages of a distributed beam-steering architecture while maintaining costs per array element at a significantly lower level than has heretofore been accomplished with that architecture.